Aluminum plates have been widely used as supports for printing plates, particularly lithographic printing plates. In conformity with variation of users' demands, the aluminum plate has shown more variety in composition, including from nearly pure aluminum with a very small content of impurities to aluminum alloys composed of aluminum as a main component. In particular, aluminum plates containing manganese have improved strength and have tended to increase in usage.
It is required for an aluminum plate to exhibit satisfactory adhesion to a photosensitive layer and water retention for use as a support for lithographic printing plates. To this effect, the surface of the aluminum plate should be roughened so as to have a uniform and dense grain. Suitability of the roughening process is a significant factor in production of printing plates, as it exerts significant influence upon the performance of a printing plate, such as stain resistance and printing durability.
Alternating current electrolytic graining is a generally employed process for roughening the surface of an aluminum plate for printing plates. Electrical current to be used in the electrolytic graining is a special alternating electric current, such as an ordinary sine wave current or a square wave current. In general, the alternating current electrolytic graining is preceded by etching treatment with an alkali, e.g., sodium hydroxide, to remove a surface layer of the aluminum plate as disclosed, e.g., in Japanese Patent Publication No. 57-16918.
However, such conventional etching treatment is unsatisfactory from the standpoint of obtaining a uniform surface roughness by the subsequent alternating electrolytic graining. This tendency is conspicuous in the case of using an aluminum plate containing manganese. More specifically, the conventional alkali etching has been effected until at least 3 g/m.sup.2 of aluminum has been removed. However, etching to such a degree does not form a uniformly etched surface. In the case of using an aluminum plate containing 0.3% or more of manganese, etc., it is particularly difficult to uniformly etch the surface due to influences of intermetallic compounds, e.g., formed between aluminum and manganese, etc. As a result, the surface cannot be roughened uniformly by the subsequent alternating current electrolytic graining, resulting in adverse influences on printed image quality. Therefore, it has been desired to develop an effective etching technique as a treatment preceding alternating electrolytic current graining treatment.